The invention relates to a process for the preparation of a catalyst suitable for use in the conversion of a mixture of carbon monoxide and hydrogen into methanol.
Among the catalysts eligible for use in the preparation on a technical scale of methanol from carbon monoxide and hydrogen, those containing copper, zinc and in addition chromium and/or aluminum are of considerable importance. The preparation of these catalysts usually comprise drying and calcining a co-precipitate obtained by adding a basic reacting substance to an aqueous solution containing salts of all the relevant metals. The catalysts thus prepared have a high selectivity towards methanol; however, both their activity and their stability are insufficient.
On the assumption that by changes in the method of preparing the co-precipitate and/or its upgrading a solution might be found to the abovementioned problem, applicants have carried out an investigation into the preparation of these catalysts. Considering; the unexpectedly favorable results which had been obtained earlier by including a spray drying step in the preparation of catalysts for the conversion of H.sub.2 /CO mixtures, it was in the first place investigated whether also in the case of the present catalysts such a step might lead to enhanced performance. Spray drying is a process which has been in use for many years for the preparation of small globular particles from a solid or a mixture of solids. The process is carried out by atomizing a dispersion in water of the material to be spray dried through a nozzle or from a rotating disc into a hot gas. The application of spray drying in the preparation of the present catalysts comprised replacing conventional drying of the aforementioned co-precipitate (which had been prepared by precipitating all the metal components together) by spray drying. This change resulted in a considerable improvement of the stability of the catalysts, which was, however, accompanied with a severe drop in activity. In view of the favorable effect spray drying had on the stability of the catalysts, further investigation was carried out to attempt to achieve improved catalysts by introducing changes in the method of preparing the co-precipitate, while maintaining the spray drying. The change in the preparation of the co-precipitate consisted in the preparation not of a single co-precipitate containing all the metals involved, but of two separate co-precipitates having different compositions. The preparation of the catalyst was carried out by dispersing the two co-precipitates together in water, spray drying the dispersion thus obtained and calcining the spray dried material. Extensive investigation during which dispersions were prepared starting from separate co-precipitates having variable compositions which were dispersed in water in different proportions to one another showed that this technique can yield catalysts having both a very high activity and a very high stability, provided that the composition of each individual co-precipitate as well as the mixing ratio of the co-precipitates in the dispersion meet the following requirements.
1. One co-precipitate (hereinafter referred to as co-precipitate (A) should contain zinc as well as chromium and/or aluminum in such quantities that the Zn/(Cr+Al) atomic ratio is 0.25-4.
2. The other co-precipitate (hereinafter referred to as co-precipitate (B) should contain copper as well as zinc and, if desired, chromium and/or aluminum in such quantities that the Cu/Zn atomic ratio is lower than 10 and the (Cr+Al)/(Cu+Zn) atomic ratio is lower than 2.
3. The two co-precipitates should be dispersed in water together and in such quantities that in the dispersion the atomic ratio of the sum of the metals coming from co-precipitate A to the sum of the metals coming from co-precipitate B is 0.25-3 and the Cu/(Cu+Zn+Cr+Al) atomic ratio is higher than 0.1.
Catalysts which, along with a very high activity, show a very high stability can only be obtained by using two separately prepared co-precipitates and spray drying a dispersion containing the two co-precipitates, if all three above-mentioned requirements are met. Any deviation from one or more of these three requirements will result in catalysts having either low activity or low stability, or a combination thereof.
The importance of including a spray drying step in the present catalyst preparation is made evident by the results of an experiment in which two separately prepared co-precipitates were mixed and the mixture was dried and calcined in the conventional manner. Although the co-precipitates used met the requirements mentioned under (1) and (2) and the mixing ratio used met the requirements mentioned under (3), the catalyst obtained showed moderate activity and extremely low stability.